The invention is related to a transmission drive belt. Such a drive belt is generally known, for example from EP-A-0.522.612, and is particularly suited for application in a continuously variable transmission or CVT for motor vehicles. The invention further relates to a motor vehicle provided with a continuously variable transmission comprising a drive belt according to the invention.
The known drive belt is suited for application in a CVT of the type provided with a primary variable pulley provided on a primary shaft and a secondary variable pulley provided on a secondary shaft. The drive belt is wound around said pulleys for transmitting torque between them. Each of the variable pulleys comprises two conical discs at least one of which is axially movable with respect to the other for, on the one hand, varying a running radius of the drive belt between the discs of a pulley and for, on the other hand, applying a pinching force to the drive belt. The amount of pinching force applied to the drive belt determines the amount of torque that can be transmitted between said shafts without slippage between drive belt and pulley occurring. The maximum amount of pinching force that may be applied is determined by the torque transmitting capacity of the drive belt, i.e. the maximum amount of torque the drive belt is allowed to be loaded with. If the torque transmitting capacity of the drive belt is exceeded, the life span of the drive belt may decrease significantly and the drive belt may fail prematurely.
The known drive belt comprises at least one continuous, i.e. circular, band slidably supported by a number of cross elements. The cross elements have a front and a back principal face separated by side faces over the thickness of the cross elements. A section of each principal face is of trapezoidal shape, the widest side of the trapezoid also being the width of the cross elements. Two sides of said section are slanted and are mutually oriented at an angle, the so called belt angle. The side faces of the cross element adjoining said section are suited and designed for co-operation with the conical pulley discs. The cross elements are usually also provided with at least one pillar part, which be connected to head part of the cross element, for forming an at least partly enclosed space accommodating the continuous band.
During operation of the drive belt, the pulley discs apply the pinching force to the side faces of the cross elements at the location of the trapezoidal section. In combination with the conical shape of the pulley discs, the pinching forces applied by the primary pulley and the secondary pulley urge the cross elements radially outward, thereby tensioning the continuous band. The torque transmitting capacity of the drive belt is related to the maximum allowable stress level in the continuous band and is thus proportional to the surface area of a cross section of the continuous band transverse to the longitudinal direction of the drive belt.
An known measure for increasing the torque transmitting capacity of the drive belt is thus to increase said surface area by increasing the radial thickness of the continuous band. However, to maintain the required flexibility and resistance to fatiguing of the drive belt, the radial thickness of the continuous band is limited. Therefore, the drive belt is usually provided with a number of continuous bands, which are radially stacked one around the other forming a laminated ring. Often two said laminated rings are adopted in a single drive belt mutually axially spaced.
Another known measure for increasing the torque transmitting capacity of a drive belt is to adopt a small contact angle between the drive belt and the pulley discs, as is described in the European patent application EP-A-0.798.492. Under the influence of a given pinching force the cross elements are urged radially outward with a force which is essentially proportionally dependent on the contact angle. Thus, when said contact angle is reduced, the continuous band is tensioned less at a given pinching force. In effect this means, that the allowable pinching force and thus the torque transmitting capacity of the drive belt may be increased by reducing said contact angle. For the drive belt this means that the belt angle, at which the two slanted sides of the trapezoidal section are mutually oriented, is to be made smaller.
Yet another known measure for increasing the torque transmitting capacity of a drive belt is to increase the axial width of the continuous band or bands. In doing so, the axial dimension, i.e. the width, of the cross elements is to be increased accordingly in order to sufficiently support the continuous band or bands. The article xe2x80x9cIs CVT the car transmission of the futurexe2x80x9d published November 1994 in the journal of Mechanical Engineering discusses drive belts with 24, 30 and 40 millimetre wide steel cross elements. Such drive belts have been manufactured with two laminated rings each formed by 9, 10 or 12 continuous bands made of maraging steel. From this article it appears that by increasing the width of the cross elements or by increasing the number of continuous bands the torque transmitting capacity of the drive belt may be increased.
In practice, however, it appears that such wider belts do not automatically result in an increased power transmitting capacity of the drive belt. Moreover, changing the contact angle goes with immense technical consequences as well as high costs throughout the chain of automotive industry, since it goes with changes in the design of pulleys, transmission control system, hydraulic pump, etc., and is consequently not desired in each case. Changing the number of continuous bands is often not preferred for logistic, manufacturing as well as cost reasons. The problem underlying. the invention encompasses the question of how to influence drive belt torque transmitting capacity within the aforementioned constraints.
The invention is thus concerned with providing a tool for drive belt within the aforementioned constraints. The invention also takes into account the material of which the cross elements are made. According to the invention the width of the cross elements and thus of the continuous band or bands may be determined depending on the specific mass of the cross elements by applying the below equation 1. Furthermore, said equation provides a simple means to determine the design of a drive belt with essentially maximum torque transmitting capacity within the constraints of a given contact angle and a given number of continuous bands. The invention makes use of the insight that during use of the drive belt the stress in the continuous band or bands depends not only on the surface area of a cross section of the continuous band or bands transverse to the longitudinal direction of the drive, but also to a significant extent on the weight of a cross element. The torque transmitting capacity of a drive belt is limited and can not be increased significantly beyond a certain value by further increasing the width of a continuous band and of the supporting cross elements. According to the invention it was surprisingly found that the torque transmitting capacity of a drive belt might actually decrease beyond a certain width of the cross elements. The technical insight is, that during operation of a continuously variable transmission equipped with a drive belt, situations occur wherein the centrifugal force acting on the drive belt is of the same order of magnitude as the forces in the drive belt due to the toque transmitted by the belt. According to the invention this results in an upper limit to the useful width of the cross elements and thus of the continuous band or bands.
This insight may be understood as follows. The torque transmitting capacity of the drive belt is substantially proportional to the width of its continuous band or bands. When said width is increased, the width, i.e. the axial dimension, of the supporting cross elements have to be increased accordingly to be able to support the continuous bands. Since the cross elements are subjected to the compressive pinching force in between the pulley discs, the height and the thickness of the cross elements is to be increased accordingly to maintain the strength of the cross elements and in particular their resistance against buckling. An increase in the thickness of the cross elements does not increase the overall mass of the drive belt, since this just means that less cross elements are needed to make up the drive belt. However, the mass of the drive belt, and therefore the centrifugal force acting on said drive belt, is still substantially proportional to the width and the height of the cross elements. From the above it may be concluded that the torque transmitting capacity of the drive belt increases substantially proportionally to the width of its continuous band or bands, and thus to the width of the cross elements, but also decreases substantially proportionally to the width and the height of the cross elements, due to the centrifugal force. As mentioned earlier, the height is assumed to increase with increasing width, so the torque transmitting capacity may be assumed to decrease substantially proportionally to the squared width of the cross elements.
It was found that, the equation for determining the width of the cross elements of the drive belt, i.e. the sum of the proportionality""s, may be approximated by:
B2xc2x7xcfx81xe2x89xa610[kg/m]xe2x80x83xe2x80x83(Equation 1)
wherein B is the width of the cross elements, xcfx81 is the specific mass of the cross elements. Actually, the boundary value is dependent on the specific automotive application of the drive belt, however, a value of about 10 kg/m was found to be a generally applicable value for automotive applications. For a drive belt with steel cross elements this means that the width B of the cross elements should be about 36 mm or less. It may thus be concluded, that whereas the known drive belts with 24 mm and 30 mm wide cross elements satisfy the equation, the drive belt with 40 mm wide cross elements will not perform as may be expected beforehand.
The invention furthermore provides for a drive belt with essentially maximum torque transmitting capacity. According to the invention such a drive belt is characterised by cross elements having a width B that satisfies the following relation:
B2xc2x7xcfx81=9.0xc2x11.0[kg/m]xe2x80x83xe2x80x83(Equation 2)
According to the invention the number of continuous bands forming a laminated ring is also bound. Said number of continuous bands may be 15 at most. It was found that the increase in torque transmitting capacity associated with adding a 16th continuous band does not overcome the accompanying drawbacks, such as the additional assembly costs, weight and frictional losses during operation.
According to the invention it is furthermore preferred to adopt cross elements wherein the two slanted sides of the trapezoidal section are oriented at a belt angle of more than 0.14 radians but less than 0.38 radians and which is preferably equal to essentially 0.26 radians. When said belt angle is smaller than 0.14 radians it becomes almost impossible to change the running radius of the drive belt and thereby the transmission ratio of the CVT, whereas when said belt angle is larger than 0.38 radians the tension if the continuous bands due to the pinching force becomes inefficiently large. A value of approximately 0.26 radians was found to be the optimum between the boundary values.
The invention is further related to a motor vehicle according to equation 4 below. With equation 4, the width B of the cross elements may be calculated for a given automotive application of the drive belt. Equation 4 may be more applicable in comparison to equation 1, for applications having a more or less extreme maximum engine speed, engine torque or belt running radius. As was explained in the above, the width B of the cross elements is dependent on the centrifugal force acting on the drive belt, because the belt load is partly determined by said centrifugal force. The centrifugal force is usually largest when the motor vehicle in which the drive belt is applied is moving at maximum vehicle speed. In more general terms, the highest centrifugal force acts on the drive belt when the quotient of the drive belt running radius and the squared longitudinal speed of the drive belt is smallest, as may be concluded from the well known equation defining the centrifugal force Fc acting on an object:                               F          c                =                              m            ·                          v              2                                r                                    (                  Equation          ⁢                      xe2x80x83                    ⁢          3                )            
wherein in m is the mass, v is the speed and r is the radius of the trajectory of the object. As is discussed earlier, the stress in the drive belt is also determined by the engine torque to be transmitted by the drive belt. Furthermore, the bending stress occurring when the drive belt runs between the discs of the pulleys in a curved trajectory also contributes to the overall stress in the drive belt. Using these insights in combination with the equation for determining centrifugal force, it is found according to the invention that the equation for determining the dependency of the width B of a cross element is given by:                                           B            2                    ·                      ρ                          (                              r                ·                                  v                                      -                    2                                                              )                                      ≤                              2000            ·                          T              MAX                                -                      1600                          r              MIN                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          4                )            